# 2600hp Hybrid Electric Turboprop regional airliner being tested

### Help Support HomeBuiltAirplanes.com:

#### Aesquire

##### Well-Known Member
I think the assumption is the turbine will be at 100% power all the time. Charging the ( not existing today but promised in 3 years ) batteries that will provide extra power for take off and climb, and tolerate fairly robust charging in cruise, descent, and taxi.

Regenerative charging on descent may be possible, but in reality not used often, depending on traffic control getting adapted to the slow down, passenger comfort in pressure rise, and battery capacity to charge rates without exploding. 3 variables I cannot quantify today.

Good luck.

A Florida utility is looking to build a giant battery pack to load level an entire grid area to allow intermittent power production. It has to be batteries because there are no mountains to pump water up to a lake & recover the power by gravity. ( as used in New York )

Imho it's a stupid, even insane, use of lithium chemistry batteries, since mass is not an issue, and the chemicals are semi-rare and expensive. Again, imho, Edison iron chemistry cells would be cheaper, not deplete a scarce resource, and will produce Hydrogen gas for co-generation or other purposes. Shipping containers on gravel pads with pallets of Edison cells, covering acres of swampy ( otherwise useless land ) connected with Hydrogen recovery pipes & bus bars, doesn't need light weight cells. It needs sturdy cells that can operate relatively unattended, he easily rebuilt by guys & gals in shirt sleeves with the only hazardous materials being the acid recovered from flushing a cell pallet for rebuilding & acid recovery recycling.

I could be wrong on the specifics! But the philosophy of appropriate tech, not using the wrong materials for fad reasons, applies.

A giant sports stadium may use carbon fiber for the solar nano film roof, but the seats and roof are on concrete.

#### markaeric

##### Well-Known Member
Passengers want a 3° descent. Which requires power.
So maybe not regen in this case, but an opportunity to sap some of that power to recharge?

#### Workhorse

##### Well-Known Member
Here is an interesting video:

#### pictsidhe

##### Well-Known Member
Regen only possibly makes theoretical sense in a motorglider with the engine off in a strong thermal. An airliner on approach with both engines off and in regen would be insanity. Passengers want a 3° descent. Which requires power.
That doesn't require full power. I know when an airliner starts its descent when I hear the engines throttled way back.
The fuel flow doesn't drop nearly as much as the thrust, so there is an effciency saving to be had from keeping the engines at full power, but cutting thrust by diverting a large chunk of power to the batteries.

#### Tiger Tim

##### Well-Known Member
Passengers want a 3° descent.
In my experience passengers want cheap tickets. If you could shave five bucks off the ticket price but had to come down at 10* you'd have a higher load factor than anyone else in the air. They may complain to all their friends about it but they'll still buy that ticket over and over again.

#### Vigilant1

##### Well-Known Member
At one time (still?) the USCG standard procedure was (is?) to shut down two engines on their C-130s when on patrol. The remaining 2 provide enough power for cruise flight, and operating them at higher power improves overall efficiency vs. 4 engines at lower output.
I'd think a setup that allowed a small commercial turboprop to shut down an engine (or two) in cruise while maintaining symmetrical thrust would be more cost effective, safer, and more flexible than this hybrid scheme.
It appears to be another feel-good way to burn "green" money.

#### BBerson

##### Well-Known Member
HBA Supporter
In my experience passengers want cheap tickets. If you could shave five bucks off the ticket price but had to come down at 10* you'd have a higher load factor than anyone else in the air. They may complain to all their friends about it but they'll still buy that ticket over and over again.
Are they asking for cheap tickets where you shut down both engines, put the props in regen mode, and dive at the runway? (and please don't accidentally get one in regen and not the other and crash)
I coast my CRX with the engine off (no point coasting with engine at idle, it cruises almost at idle)
But coasting takes skill and is risky (and illegal).

#### Tiger Tim

##### Well-Known Member
Are they asking for cheap tickets where you shut down both engines, put the props in regen mode, and dive at the runway?
It's still early and too hush-hush to know for sure but I expect that shutting engines off and 'switching modes' are both very much off the table. From what I've read, I picture these hybrid systems being essentially a free turbine where the propeller gearbox (power section, is P&W terms) is dual driven by the hot gases from the compressor section and by a motor/generator. I would expect power delivery in flight to be automatically managed by the airplane, but in essence the movement of the power lever gives a linear chance in total torque at the prop where the bottom end of the range is all fuel-burning turbine until it maxes out at probably cruise torque, then above that power gets added by the motor. At exceptionally low power settings in a steep descent, once the prop starts to windmill, rather than triggering an auto-feather like current turboprops it would leave the prop in fine-ish pitch and use that RPM to regenerate some power. A 4000-5000fpm descent is no big deal in a pressurized cabin.

At least that's how I would do it, but then again I'm no engineer.

#### BBerson

##### Well-Known Member
HBA Supporter
I suppose you could shut off the fuel and let them spin. But that would shock cool and add cycle fatigue, so some fuel would need to keep it hot.

#### Tiger Tim

##### Well-Known Member
I suppose you could shut off the fuel and let them spin. But that would shock cool and add cycle fatigue, so some fuel would need to keep it hot.
I wonder if you could have the turbine set to a much lower idle than most since there will be supplemental electrical power to prevent the bogging and lagging that’s avoided with a fast idle?

#### BBerson

##### Well-Known Member
HBA Supporter
Are you talking about bogging on the ground?
Idle in flight is different.

#### Dan Thomas

##### Well-Known Member
In my experience passengers want cheap tickets. If you could shave five bucks off the ticket price but had to come down at 10* you'd have a higher load factor than anyone else in the air. They may complain to all their friends about it but they'll still buy that ticket over and over again.
Two problems with that. First, a steeper approach means you had to fly farther in level cruise to reach the initial descent point. That doesn't sound like an energy-saving proposition. Second, the steeper approach means cabin altitude has to decrease much more quickly, and eardrums suffer. Pax hate that.

#### rickofudall

##### Member
Don't know how much effect it will have on a turbine electric airplane but I wonder about the effects on the airframe of always landing at maximum weight and lower MTOW. If you can't burn off fuel to lighten the plane then MTOW has to go down.

Rick

#### Tiger Tim

##### Well-Known Member
Are you talking about bogging on the ground?
Idle in flight is different.
Is it not the same underspeed governor handling both? It’s been a while since I had my head in the free turbine books, my experience is mostly with Garretts which behave a little differently.

First, a steeper approach means you had to fly farther in level cruise to reach the initial descent point. That doesn't sound like an energy-saving proposition. Second, the steeper approach means cabin altitude has to decrease much more quickly, and eardrums suffer. Pax hate that.
In my experience, that longer cruise portion often pays for itself in gains of true airspeed as well as better SFC and (half the time) strong tailwinds up high. I watch my planned versus actual fuel pretty closely and that technique routinely saves gas. As for the descent rate, I’ll often come down at 3000+fpm and a cabin rate of 700fpm or so with no cabin-catching issues or discomfort for anyone. With a good max differential pressure (ie. lower cabin altitude in cruise) you could come down even steeper in perfect passenger comfort.

I’m sceptical of this whole hybrid airliner scheme too but I do think on some specific flight profiles there’s will be efficiency gains with it. Thirty percent is a bold statement but I’m sure there’s something there at least, even if small. I suppose that’s the whole point of testing this contraption, to verify the math they’ve done, right?

#### BBerson

##### Well-Known Member
HBA Supporter
I don't have any turbine experience. Or regen experience in aviation. But I have coasted from the top of the Rocky Mountain Continental Divide pass in my CRX. It can maintain 60-70 mph coasting with the engine off, but if I tried to drag it down with regen it couldn't possibly maintain speed.
I don't think hybrid cars are getting much regen while going down mountain passes, and automobiles are much more efficient than any aircraft.

#### pictsidhe

##### Well-Known Member
A lot of you guys are missing the fundamental point.

Turbines drink fuel when idling. Not as much as full throttle, but its a large chunk of it.

On the descent, instead of idling them, you'd keep them spinning at a fair proportion of max power and use that to charge the batteries. Fuel usage over idling them isn't as big you expect.

#### Vigilant1

In round numbers, a PT6 will burn about 120 pph in idle at fl250, so about 18 gph. A typical descent from typical cruise for a DASH-8 might take 10 minutes, so we'll burn 3 gallons per engine in that time, or 6 gallons total. Now, we are getting something for this-- bleed air for the air conditioning, electricity and hydraulics, etc. That 6 gallons per flight costs about $25. And, the true fuel savings for this hybrid setup will be much less than that (during descent the engines will still be running, but powering generators with their own losses, more losses on the return trip from thbe batteries to the electric motors, the fuel costs of lifting those batteries and the electric motors, etc on every flight, etc). Add in the increased maintenance costs (battery replacement, more transmission parts, etc), increased capital costs (for the entire hybrid power systemi), and it is a clear loser--which is exactly the reason it isn't being done unless it uses "funny money" or subsidies. #### gtae07 ##### Well-Known Member I’m not seeing the benefit with typical aircraft operations... unless they plan to do some real short-field operations or something. That’s where I think a hybrid solution could make a lot of sense—cases where you really need a whole lot of power, but only for a couple of minutes. Imagine you could triple or quadruple the power output of a typical GA plane (or engage your VTOL lift props) for two or three minutes, to get into or out of a really short space, at a fair bit less weight penalty than an engine sized for two or three times the power output or the mechanical complexity of variable-pitch, tilting, or stowable rotors. That sort of setup would also lend itself well to a backup power source for engine-out situations. Imagine you could get five minutes of “backup power” after you lost your hydrocarbon-burning engine, enough to stretch a glide or put yourself down at an airport or in a good spot instead of just the best you could find? #### markaeric ##### Well-Known Member In round numbers, a PT6 will burn about 120 pph in idle at fl250, so about 18 gph. A typical descent from typical cruise for a DASH-8 might take 10 minutes, so we'll burn 3 gallons per engine in that time, or 6 gallons total. Now, we are getting something for this-- bleed air for the air conditioning, electricity and hydraulics, etc. That 6 gallons per flight costs about$25. And, the true fuel savings for this hybrid setup will be much less than that (during descent the engines will still be running, but powering generators with their own losses, more losses on the return trip from thbe batteries to the electric motors, the fuel costs of lifting those batteries and the electric motors, etc on every flight, etc). Add in the increased maintenance costs (battery replacement, more transmission parts, etc), increased capital costs (for the entire hybrid power systemi), and it is a clear loser--which is exactly the reason it isn't being done unless it uses "funny money" or subsidies.

It's not about what's saved during descent, but the possibility of running a turbine at full power during cruise. The motors augment available power during climb.